Broad-area laser diodes are widely used today as various pump sources for fiber lasers and optically pumped solid state lasers. The efficient coupling between broad-area laser diodes and secondary gain media, such as fiber lasers and optically pumped solid state lasers, depends on the lateral mode structure (the mode structure in a direction transverse to the direction of emission) of the radiation in the optical cavity of the broad-area laser diode and the long-term stability of these lateral modes (the modes in the direction transverse to the emission direction). A precise knowledge of the relative intensity and wavelength of the lateral modes is useful to assure the optimum light delivery to the active medium.
A lateral mode intensity monitoring device based on conventional optical instruments such as gratings and interferometers requires an optical table and a significant amount of space. This is especially true for modern long-cavity broad-area laser diodes, where the longitudinal mode frequency difference (the difference in the frequency of modes in the direction of emission) becomes comparable to the intrinsic linewidth of the broad-area laser diode, in a range of from about 1 MHz to about 30 MHz. An alternative way of monitoring the lateral modes by coherent heterodyning (using coherent external optical sources at various frequencies to create beat frequencies) requires external optical sources that also require an optical table and a significant amount of space.